1. Field of the Invention
The present invention is related to an electronic exposure control apparatus of a single lens reflex camera, particularly to an electronic exposure control apparatus having an electronic shutter formed by a digital circuit which is digitally extended using the register.
2. Description of the Prior Art
In a conventional single lens reflex camera when automatic exposure control is performed by TTL photo-sensing, since photo-sensing is suspended during the exposure of camera, as is well known, the brightness of an object just before exposure is detected and memorized and exposure control is performed based on this memorized value. For employment in such a device, that a capacitor is used as a means to memorize the brightness of an object and the analogue amount of charge in terms of voltage is memorized by the capacitor. In another type of device, a register formed by flip-flop circuits is used to memorize the brightness as a digital amount.
In the former method in which a capacitor is used to memorize analogue amount, the brightness of an object being electrically converted is compressed to several volts or less using a compression diode and is memorized with the use of a power source battery which is ordinarily used. Particularly in such device, a circuit to convert the memorized voltage to exposure time is an analogue circuit using a semi-conductor. Therefore, the device has certain shortcomings in that it is apt to be influenced by temperature variation and variation in power source voltage and the memorized voltage cannot be converted to proper exposure time. In addition even when a compensation circuit, etc. is provided, this influence cannot be sufficiently compensated.
On the other hand, a method to digitally memorize object information using a register is disclosed in the Japanese Patent Publication No. Sho 45-4903, and in this method, as the object information is digitally memorized in a register, it will not be influenced by temperature variation and power source voltage variation. However, since the object information is memorized in a register in a compressed manner for reducing the number of bits in the register, the output of the register needs to be expanded to convert what is memorized in the register to exposure time. But, since a circuit for this expansion purpose is an analogue circuit, it is apt to be affected by temperature variation, and power source voltage variation as in the former case, and it is difficult to obtain correct exposure time.